Forty-six patients with cystic fibrosis (CF) were studied for colonization and sensitization by Aspergillus organisms. The fungus was cultured from 21 of 37 (57%) patients who produced sputum. The non-CF sputum isolation rate was 6.5%. Sputum hyphae were seen in 95% of patients with positive cultures. Sputum eosinophilia accompanied colonization by Aspergillus. Aspergillus precipitins were found in 37% of CF patients; positive immediate skin tests were found in 39%. "Atopy" defined by skin test criteria was found in 46%. All of these findings were more common in patients with more severe disease (p less than 0.05). CF patients had predominant mold sensitivity in contrast to predominant pollen sensitivity in a control group of asthmatic subjects. Serum IgE concentrations were increased in 22% of CF patients. CF patients who had increased serum concentrations of IgE were more likely to have a family history of atopy. Wheezing was more common in those who were atopic, especially the group with predominant mold sensitivity who had more severe disease. Four patients fulfilled the diagnostic criteria for allergic bronchopulmonary aspergillosis (ABPA) during the time of study and follow-up. A fifth patient had ABPA diagnosed before the study. The incidence of ABPA during a 2-yr period in our CF population was 11%.